Conventionally, an exhaust gas cleaner including a particulate filter that collects particulates (hereinafter also referred to as PM), and the like has been provided in an exhaust system of a vehicle. Although PM contained in exhaust gas is collected in the particulate filter when the exhaust gas passes through a DPF, since PM deposits on the DPF in this process, thereby generating a pressure loss, fuel consumption deteriorates. Therefore, it is necessary to periodically or continuously remove PM from the DPF.
PM is derived mainly from fuel, and is composed of a soluble organic fraction (SOF), which is an easily combusted component, and soot, which is a difficult component to combust. Although the exhaust gas temperature is typically in the range of 200° C. to 450° C., the soluble organic fraction has a combustion initiation temperature of 200° C. to 550° C. and is efficiently removed; however, soot has a combustion initiation temperature of 550° C. to 700° C. and remains mostly not removed.
Therefore, fuel is periodically injected into the exhaust system and combusted, and the particulate filter are raised in temperature up to the combustion temperature of soot by the combustion heat generated at this time, thereby performing a forced regeneration process. However, in the regeneration process, it is necessary to maintain high temperature conditions of about 600° C. for 10 to 20 minutes in order to sufficiently restore the pressure loss level, and thus there is concern over fuel consumption deterioration and wear on the particulate filter. Therefore, there is a necessity to reduce the combustion initiation temperature of PM, and particularly of soot.
Techniques using a variety of catalysts that cause the combustion initiation temperature to decrease have been developed in response to such a demand. For example, an exhaust gas cleaner produced by coating a DPF in an aqueous slurry containing a catalyst material composed of silver, copper, oxides thereof, and ceria, drying, and calcining (dip-coating method) has been disclosed in Patent Document 1.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2001-73748